Position manipulators are employed in a vast array of applications to position objects, tool, or instruments with varying degrees of precision. A survey of kinematic joints, or kinematic pairs, that may be used in position manipulators are illustrated in FIG. 1, including: rigid (no motion), prismatic, revolute, parallel cylinders, cylindrical, spherical, planar, edge slider, cylindrical slider, point slider, spherical slider, and crossed cylinder.
The Stewart Platform (also referred to herein as a hexapod) is a multi-axis positioning stage made up of six actuators with spherical, ball, or universal joints at both ends of each actuator, for example. Hexapods are considered the world class multi-axis positioning stage design for most applications, but are often cost-prohibitive. One problem with hexapods is that it is a synergistic motion platform because of the mutual interaction of the actuators. That is, due to the mutual interaction of the actuators, none of the actuators can be moved independently; a given move requires many or all of the actuators to move different specific amounts and at different speed profiles to prevent the stage from binding. Additionally, these motion and speed profiles change continuously as the defined starting and ending points are changed. For this reason, a highly complex computer algorithm is required to individually calculate the distance to travel and speed profiles necessary for each actuator to get the top plate of the stage from point A to point B, even if a short distance single axis move is desired. As a result, a human operator is incapable of manually performing, even this simple move, without binding the stage.
Another significant disadvantage with a hexapod is that the stiffness of the joints (against off axis motion) dictates the “slop,” or “play,” and, therefore, the resolution of the stage. This is a design conflict because it is exponentially more difficult to make spherical joints (employed in hexapods) at tighter and tighter tolerances. That is, in the case where a designer makes a world class spherical bearing to maximize stage resolution and minimize slop, he has, by default, exacerbated two inherent issues. First, because of the rigidness of the spherical joints, the accuracy of the motion and speed profile requirements for each actuator increases exponentially to prevent binding. Second, the capability requirements of the actuators increases exponentially in order to achieve the required precision motions and speed profiles. As a result, improving the resolution of a hexapod requires an exponential increase in computing power for determining motion and speed profiles, an exponential increase in the performance capabilities of the actuators, and twelve high quality spherical bearings. All of these factors drive up the cost of a hexapod significantly.
Although hexapods typically cost from three to ten times as much as their kinematic chain counterpart, they are often preferred because they do not suffer from tolerance stack up issues. Ten microns of precision is not an uncommon positioner requirement for many applications and, for example, in the photonics industry, submicron precision is often required. At this date, hexapods typically cost from $60,000 to greater than $120,000, each depending on physical size, load limits, and precision requirements. An alternative precise position manipulator would be highly desirable.